A new generation of membranes suited for membrane chromatography and a variety of other applications including microfiltration, ultrafiltration, membrane absorbers, etc. has been developed. The membranes are useful in e.g. bio-separations where they can be used as disposable devices that have superior performance to existing separation systems.
These membranes comprise a macroporous gel anchored within a suitable non-woven support. In order to achieve high protein and other molecule adsorption capacities in chromatography and related applications, the gels are typically relatively soft, thereby allowing diffusion of the target molecules into the gel phase. Using this approach, dynamic capacities that are a factor of six higher than the best alternative membrane products can be achieved.
There is a downside to using soft macroporous gels. This comes from the fluxes that can be achieved with these membranes, particularly when the membranes are combined to form a stack as is typically used in membrane chromatography applications. Ideally, the flux drop at a given pressure should decrease linearly as additional membranes are added to a stack. It is well known that the flux through a membrane is inversely proportional to the thickness of the active layer. For example, whereas the flux of a single Q-type membrane at 100 kPa is 2500 kg/m2h when eight layers are combined, the resulting flux is 135 kg/m2h, rather than the expected value of 2,500/8=312 kg/m2h. This reduction in flux can be offset by increasing the trans-membrane pressure. But this comes with the added cost of more expensive membrane housings, pumps, etc.
The problem becomes greater in higher capacity gel based membranes with softer gels. Stacks of these membranes exhibit a non-linear pressure flux relationship. In severe cases, the flux reaches a limiting value as the pressure is increased. This effect limits the thickness of the membrane stacks that can be used if high flow rates are to be maintained.
It is known to use certain types of spacers between layers in membrane stacks for particular purposes.
Spacers are common elements in membrane systems or modules used in desalination processes such as reverse osmosis, electrodialysis, electro-deionization and diffusion dialysis. In these systems a “spacer” is a device that provides a generally defined distance between two adjacent membrane sheets to allow cross-flow of a liquid between the two membrane sheets. A feed fluid spacer is a porous spacer layer that provides for the passage of the feed fluid over and parallel to the active side of a membrane. This flow parallel to a membrane surface is termed cross-flow. The feed fluid spacer serves the function of directing the feed fluid to cover the active side of the membrane in a uniform manner. The spacer may also impart turbulence to the feed fluid to provide good mixing in the feed fluid as it travels over the active side of the membrane and to reduce concentration polarization.
A typical spiral wound membrane module comprising such spacers is shown in FIG. 1.
The spiral module contains tightly packed membranes sandwiched between mesh spacers and wrapped around a small-diameter central tube. There are two spacers in this module; a permeate carrier layer as well as a feed spacer/distributor.
The membrane stack includes two, long semipermeable membranes with a spacer mesh between them. This is then wound up in a spiral tube with another spacer, the permeate carrier layer, to separate the outer, permeate sides of the stack. The mesh separator or spacer on the feed side allows the feed to be forced in one side of the spiral cylinder and out the other side. Pressure on the feed side forces some of the water (typically less than 15% of the feed water passes through the membrane on a single pass) to pass through the membrane where it is collected in the space between the membranes. The resulting permeate then flows around the spiral where it is collected in the centre of the tube. An early description of a spiral wound device is given in Bray D. T. Reverse osmosis purification apparatus. U.S. Pat. No. 3,417,870. 1968.
The key point in this design of a membrane module is that the spacers on the feed and permeate side allow flow of the two streams parallel to the surfaces of the membranes.
Another common example of membrane stacks is in electrodialysis cells. Electrodialysis is a process that uses a direct electrical current to remove salt, other organic constituents, and certain low molecular weight organics from brackish water. With this technique several hundred flat, ion permeable membranes and water flow spacers are vertically assembled in a stack. Half of the membranes allow positively charged ions, or cations, to pass through them. The other half-anion-permeable membranes—allow negatively charged ions to pass through them. The anion permeable membranes are alternately placed between the cation-permeable membranes. Each membrane is separated from the adjacent membrane in the stack by a polyethylene flow spacer.
An electrical current is established across the stack by electrodes positioned at both ends of the stack. Brackish water is pumped at low pressures into the flow spacers between each membrane and passes through the cell exiting on the opposite side. While passing through the cell, ionic constituents are electrochemically driven through the membranes on either side of the channel. This results in removal of salts from the brackish water stream and the formation of a concentrated salt steam in the intervening channels. The spacers serve to hold the membranes in place and allow the flow of the brackish water and resulting more concentrated streams parallel to the membrane surfaces. Typical stack designs are shown in the patents of Iaconelli (U.S. Pat. No. 3,695,444) and Olsen (U.S. Pat. No. 3,623,610).
It will be appreciated from the above that spacers are conventionally used to permit fluid to flow in the plane of the major surface of a membrane. The spacers used in such devices as spiral wound modules where tangential flow occurs across-the surface of the membranes from a feed channel containing the spacer, are typically quite thick and constructed such that flow channels remain even when sandwiched tightly between two membranes. This flow path is constructed to be tortuous such that movement of the feed through the spacer is turbulent. The design of these spacers is complex with different sized elements making up the mesh like material so as to provide the tortuous path.
But the art does not disclose the use of spacers in instances where the fluid flow path is substantially perpendicular to the plane of a major surface of the membrane layers in a membrane stack.